Chads take on acid/base

[OU11BB]

i just watched chads video over acid/base. he uses the cardio mnemonic to solve his problems. Does this approach work for all of these types of problems and especially what will be seen on the DAT? Also if anyone knows any exceptions to this approach could you please briefly share. thank you

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[Cool Beans]

i just watched chads video over acid/base. he uses the cardio mnemonic to solve his problems. Does this approach work for all of these types of problems and especially what will be seen on the DAT? Also if anyone knows any exceptions to this approach could you please briefly share. thank you

Hey OU11BB,
The CARDIO mnemonic is effective but not full proof (no set of rules is).
Charge
Atom
Resonance
Dipole Induction
Orbital

As a general rule, it should lead you to the right answer >95% of the time. Notable exceptions I've seen involve the 7 strong acids: HCl is not positively charged whereas NH4+ (or other quaternary ammonium ions) are. The Charge rule states that a more positively charged species tends to be more acidic. But obviously HCl, a strong acid, is way more acidic than NH4+. The same problem is encountered with carboxylic acids. Even though carboxylic acids (pKa~5) are not charged and NH4+ (pKa~9) is, the carboxylic acid is definitely more acidic.

The other notable exception (much less common though) I've seen is to the Atom rule. A terminal alkyne (pka~26) is more acidic than ammonia (pka~35) even though nitrogen is more electronegative than carbon. The difference here is that the carbon in a terminal alkyne is sp hybridized whereas the nitrogen in an amine is sp3 hybridized. This is why NaNH2 is used to deprotonate a terminal alkyne:
R-C=C-H + NaNH2-->R-C=C-Na+ + NH3

Hope this helps!